Although molybdenum has been supported as a specific heat standard material by the NIST for quite a while, adequate information is not always available regarding its properties, including thermal conductivity, thermal diffusivity, and thermal expansion.
In accordance with literature, pure molybdenum should not exhibit any phase changes until it reaches its melting point. This characteristic is crucial considering oxygen sensitivity of molybdenum at elevated temperatures. Owing to the high vapor pressure of molybdenum oxides, the properties of the material does typically not change due to surface oxidation and the oxides formed are simply evaporated from the surface. All these exotic properties make molybdenum a good element for a multi-property standard material.
In this experiment, a pure (99.99%) molybdenum material was analyzed to measure different thermophysical properties. The density change and thermal expansion were measured using pushrod dilatometry (DIL), while the specific heat was measured using differential scanning calorimetry (DSC) and the laser flash technique (LFA) was used to determine the thermal diffusivity.
Tests were performed on different samples created from the original block and measured between the temperature range of -125°C and 1400 °C. This allowed the material to be assessed as a potential candidate for a standard material for various thermophysical properties over a wide range of temperatures. For the different test methods, many different samples were prepared from the cylinder block. Two samples were prepared and evaluated two to three times for each measurement technique. The homogeneity and thermal stability of the material were observed and the repeatability of the test results was ascertained.
The measured thermal expansion results for the two different molybdenum samples are illustrated in Figure 1. Data scattering between the samples and the various tests are typically within ±1.5%. Taking into account the impact of evaporation of oxides, the influences of surface effects, and the accuracy and repeatability of the equipment used, data scattering is in an acceptable range. Material inhomogeneities or variations in the thermal expansion values between the various heating runs were not observed from the results.
Figure 1. Thermal expansion (DIL 402 °C)
The density change and the volumetric expansion of molybdenum versus temperature are shown in Figure 2. The volumetric expansion was identified from the measured thermal expansion. The expansion behavior is same in all directions because of the assumption of an isotropic behavior of the material. Density was calculated on the basis of the volumetric expansion and the room-temperature bulk density of 10.216 gcm-3. The room-temperature bulk density was calculated by measuring the mass and volume of the originally supplied sample block.
Figure 2. Volumetric expansion and density change (DIL 402 C and Density Determination software)
The measurements of the specific heat values by DSC are depicted in Figure 3. The variation between individual results was within ±2.0%, which is by far within the specified uncertainty of the equipment used for the tests. A strong increase in values was observed in the low-temperature range, agreeing well with the well-known Debye theory. The increase in values was nearly linear at elevated temperatures, agreeing well with the solid state physics. No overlapping transition or other thermal effects were observed within this temperature range, clearly showing that the material does not undergo any phase change between -125 °C and 1275 °C. This meets the requirement as a standard material because no structural changes in the temperature range of interest take place.
Figure 3. Specific heat (DSC 404 Pegasus)
The thermal diffusivity measurement results from the various flash devices employed for the tests are illustrated in Figure 4. There was a decrease in the thermal diffusivity versus temperature. The decrease is in line with the T-1 behavior below 600 °C, causing a virtually linear decrease at elevated temperatures. This is probably due to the small contribution by the electrons for this metallic material. Data scattering from sample to sample and run to run is typically within ±2%, and slightly higher scattering (±3%) occurred at 1000 °C. This is probably due to the evaporation of molybdenum oxides at this temperature range, affecting the samples emissivity and consequently the absorption of laser light and emission of infrared light.
Figure 4. Thermal diffusivity (LFA 457 MicroFlash)
The thermal conductivity results obtained as the product of the measured thermal diffusivity, specific heat, and density are depicted in Figure 5. By linearly extrapolating the measured data, the specific heat beyond 1275 °C and the density data below room temperature were determined. The results showed that the thermal conductivity follow the temperature dependence of the thermal diffusivity.
Figure 5. Thermal conductivity | (λ = a(T)•cp(T)•ρ(T)
Different thermophysical properties of high-purity molybdenum were determined and compared with the literature values. The comparison corroborated the quality of the measurement results and the reliability of the material. The test results assessed that pure molybdenum might be a potential candidate to be utilized as a standard material up to elevated temperatures beyond 1200 °C. It also holds potential to be used as a calibration standard for different thermophysical properties. Further analyses at many different laboratories and test institutes would be appreciated to corroborate the potential of the material.
This information has been sourced, reviewed and adapted from materials provided by NETZSCH-Gerätebau GmbH.
For more information on this source, please visit NETZSCH-Gerätebau GmbH.